26,625 research outputs found
Dissipative Bose-Josephson junction coupled to bosonic baths
We investigate the effect of dissipation in a Bose-Josephson junction (BJJ)
coupled to bath of bosons at two sites. Apart from the dynamical transition due
to repulsive interactions, the BJJ undergoes a quantum phase transition by
increasing the coupling strength with the bath modes. We analyze this system by
mapping to an equivalent spin model coupled to the bosonic modes. The
excitation energies and fluctuation of number imbalance are obtained within
Holstein-Primakoff approximation, which exhibit vanishing of energy gap and
enhanced quantum fluctuations at the critical coupling. We study the dynamics
of BJJ using time dependent variational method and analyze stability of
different types of steady states. As a special case we study in details the
phase space dynamics of BJJ coupled to a single mode, which reveals diffusive
and incoherent behaviour with increasing coupling to the bath mode. The
dynamical steady states corresponding to the Pi-oscillation and self-trapped
state become unstable in the region where their oscillation frequencies are in
resonance with the bath modes. We investigate the time evolution of number
imbalance and relative phase in presence of Ohmic bath with Gaussian noise to
incorporate thermal fluctuations. Apart from damping of Josephson oscillations
and transition to symmetry broken state for strong coupling we observe decay of
Pi-oscillation and self-trapped state to the ground state as a result of
dissipation. Variation of phase fluctuation with temperature of the bath shows
similar behaviour as observed in experiment. Finally we discuss the
experimental setup to study the observable effects of dissipation in BJJ
Stress controlled magnetic properties of Cobalt nanowires
We investigate the magnetic properties of a composite comprising of
ferromagnetic Cobalt nanowires embedded in nanoporous anodized alumina
template. We observe unusual increase in, the saturation magnetization and the
coercive field, of the nanowires below 100 K. We also report the appearance of
an unusual exchange bias effect in nanowires below 100 K. We argue our results
can be understood on the basis of a competition between different magnetic
energy scales induced by significant stresses acting on the nanowires at low
temperatures. The composite behaves as an effective medium in which the
magnetic anisotropy of nanowires can be conveniently controlled via stress on
the nanowires.Comment: 16 pages, 7 figures, Submitte
Controlling spatiotemporal chaos and spiral turbulence in excitable media: A review
Excitable media are a generic class of models used to simulate a wide variety
of natural systems including cardiac tissue. Propagation of excitation waves in
this medium results in the formation of characteristic patterns such as
rotating spiral waves. Instabilities in these structures may lead to
spatiotemporal chaos through spiral turbulence, which has been linked to
clinically diagnosed conditions such as cardiac fibrillation. Usual methods for
controlling such phenomena involve very large amplitude perturbations and have
several drawbacks. There have been several recent attempts to develop
low-amplitude control procedures for spatiotemporal chaos in excitable media
which are reviewed in this paper. The control schemes have been broadly
classified by us into three types: (i) global, (ii) non-global
spatially-extended and (iii) local, depending on the way the control signal is
applied, and we discuss the merits and drawbacks for each.Comment: 9 pages, 6 figures; A version of the work will appear as Chapter 32
in Handbook of Chaos Control, 2nd Revised edition, (Eds.) E Scholl and H G
Schuster, Wiley-VCH, Berlin (2007
A finite temperature study of ideal quantum gases in the presence of one dimensional quasi-periodic potential
We study the thermodynamics of ideal Bose gas as well as the transport
properties of non interacting bosons and fermions in a one dimensional
quasi-periodic potential, namely Aubry-Andr\'e (AA) model at finite
temperature. For bosons in finite size systems, the effect of quasi-periodic
potential on the crossover phenomena corresponding to Bose-Einstein
condensation (BEC), superfluidity and localization phenomena at finite
temperatures are investigated. From the ground state number fluctuation we
calculate the crossover temperature of BEC which exhibits a non monotonic
behavior with the strength of AA potential and vanishes at the self-dual
critical point following power law. Appropriate rescaling of the crossover
temperatures reveals universal behavior which is studied for different
quasi-periodicity of the AA model. Finally, we study the temperature and flux
dependence of the persistent current of fermions in presence of a
quasi-periodic potential to identify the localization at the Fermi energy from
the decay of the current.Comment: 25 pages, 12 figure
Phases and collective modes of hardcore Bose-Fermi mixture in an optical lattice
We obtain the phase diagram of a Bose-Fermi mixture of hardcore spinless
Bosons and spin-polarized Fermions with nearest neighbor intra-species
interaction and on-site inter-species repulsion in an optical lattice at
half-filling using a slave-boson mean-field theory. We show that such a system
can have four possible phases which are a) supersolid Bosons coexisting with
Fermions in the Mott state, b) Mott state of Bosons coexisting with Fermions in
a metallic or charge-density wave state, c) a metallic Fermionic state
coexisting with superfluid phase of Bosons, and d) Mott insulating state of
Fermions and Bosons. We chart out the phase diagram of the system and provide
analytical expressions for the phase boundaries within mean-field theory. We
demonstrate that the transition between these phases are generically first
order with the exception of that between the supersolid and the Mott states
which is a continuous quantum phase transition. We also obtain the low-energy
collective excitations of the system in these phases. Finally, we study the
particle-hole excitations in the Mott insulating phase and use it to determine
the dynamical critical exponent for the supersolid-Mott insulator
transition. We discuss experiments which can test our theory.Comment: 10 pages 6 Figs v2: Updated version with more refs and additional
discussion
Emergence of two-phase behavior in markets through interaction and learning in agents with bounded rationality
Phenomena which involves collective choice of many agents who are interacting
with each other and choosing one of several alternatives, based on the limited
information available to them, frequently show switching between two distinct
phases characterized by a bimodal and an unimodal distribution respectively.
Examples include financial markets, movie popularity and electoral behavior.
Here we present a model for this biphasic behavior and argue that it arises
from interactions in a local neighborhood and adaptation & learning based on
information about the effectiveness of past choices.Comment: 5 pages, 2 figures, to appear in "Practical Fruits of Econophysics",
Proc. 3rd Nikkei Econophysics Symposium, Tokyo, Nov 2004 (Springer
Signature of Chaos and Delocalization in a Periodically Driven Many Body System : An Out-of-Time-Order Correlation Study
We study out-of-time-order correlation (OTOC) for one-dimensional
periodically driven hardcore bosons in the presence of Aubry-Andr\'e (AA)
potential and show that both the spectral properties and the saturation values
of OTOC in the steady state of these driven systems provide a clear distinction
between the localized and delocalized phases of these models. Our results,
obtained via exact numerical diagonalization of these boson chains, thus
indicate that OTOC can provide a signature of drive induced delocalization even
for systems which do not have a well defined semiclassical (and/or large N)
limit. We demonstrate the presence of such signature by analyzing two different
drive protocols for hardcore bosons chains leading to distinct physical
phenomena and discuss experiments which can test our theory
Quantum Signature of Chaos and Thermalization in Kicked Dicke Model
We study the quantum dynamics of the kicked Dicke model(KDM) in terms of the
Floquet operator and analyze the connection between the chaos and
thermalization in this context. The Hamiltonian map is constructed by taking
the classical limit of the Heisenberg equation of motion suitably to study the
corresponding phase space dynamics which shows a crossover from regular to
chaotic motion by tuning the kicking strength. The fixed point analysis and
calculation of the Lyapunov exponent(LE) provides us a complete picture of the
onset of chaos in phase space dynamics. We carry out the spectral analysis of
the Floquet operator which include the calculation of quasienergy spacing
distribution, structural entropy and show the correspondence to the random
matrix theory in the chaotic regime. Finally, we analyze the thermodynamics and
statistical properties of the bosonic sector as well as the spin sector and
discuss how such periodically kicked system relaxes to a thermalized state in
accordance with the laws of statistical mechanics. We introduce the notion of
an effective temperature and show that a microcanonical picture is emerging out
in the thermodynamic limit indicating the thermalization occurring in such
system.Comment: 10 pages, 10 figure
Drive Induced Delocalization in Aubry-Andr\'e Model
Motivated by the recent experiment by Bordia et al [Nat. Phys. 13, 460
(2017)], we study single particle delocalization phenomena of Aubry-Andr\'e
(AA) model subjected to periodic drives. In two distinct cases we construct an
equivalent classical description to illustrate that the drive induced
delocalization phenomena stems from an instability and onset of chaos in the
underlying dynamics. In the first case we analyze the delocalization and the
thermalization in a time modulated AA potential with respect to driving
frequency and demonstrate that there exists a threshold value of the amplitude
of the drive. In the next example, we show that the periodic modulation of the
hopping amplitude leads to an unusual effect on delocalization with a
non-monotonic dependence on the driving frequency. Within a window of such
driving frequency a delocalized Floquet band with mobility edge appears,
exhibiting multifractality in the spectrum as well as in the Floquet
eigenfunctions. Finally, we explore the effect of interaction and discuss how
the results of the present analysis can be tested experimentally
Phases, collective modes, and non-equilibrium dynamics of dissipative Rydberg atoms
We use a density matrix formalism to study the equilibrium phases and
non-equilibrium dynamics of a system of dissipative Rydberg atoms in an optical
lattice within mean-field theory. We provide equations for the fixed points of
the density matrix evolution for atoms with infinite on-site repulsion and
analyze these equations to obtain their Mott insulator- superfluid (MI-SF)
phase boundary. A stability analysis around these fixed points provides us with
the excitation spectrum of the atoms both in the MI and SF phases. We study the
nature of the MI-SF critical point in the presence of finite dissipation of
Rydberg excitations, discuss the fate of the superfluidity of the atoms in the
presence of such dissipation in the weak-coupling limit using a coherent state
representation of the density matrix, and extend our analysis to Rydberg atoms
with finite on-site interaction via numerical solution of the density matrix
equations. Finally, we vary the boson (atom) hopping parameter and the
dissipation parameter according to a linear ramp protocol. We study
the evolution of entropy of the system following such a ramp and show that the
deviation of the entropy from its steady state value for the latter protocol
exhibits power-law behavior as a function of the ramp time. We discuss
experiments which can test our theory.Comment: v2 10 pages 8 figs; minor typos correcte
- …